1. Field of the Invention
The present invention relates to a filter bank composed of a plurality of ,surface acoustic wave filters having different pass bands.
2. Description of the Prior Art
FIG. 1 is a circuit diagram showing a conventional filter bank illustrated in 1975 Ultrasonics Symposium Proceedings, IEEE Cat. #75 CHO 994-4SU pp. 311-314. In the diagram, the numeral 1 shows a plurality of surface acoustic wave filters having different pass bands, which are provided on a surface of a piezo-electric substrate to perform conversion between electric signals and surface acoustic waves. These filters are connected using an interdigital transducer 2-a as a shunt element and an inductor 3 as a series element on each input side in the form of ladders with each other. An output terminal of an interdigital transducer 2-b on the output side of each surface acoustic wave filter 1 forms each output terminal 4 of a filter bank, to which each load resistor 5 is connected. The numerals 6, 7, and 8 show a source resistance, a signal source, and a termination resistance, respectively.
Next, the operation will be described.
The filter bank shown in FIG. 1 has a construction which each T-type circuit comprising a series element, that is, two series inductors 3 and a shunt element, that is, an interdigital transducer 2-a is cascade connected. Here, in order to make the operation of the filter bank simple, these will be given a description of characteristics of one section of the T-type circuit shown in FIG. 2. At the frequency for which the surface acoustic wave filter 1 forms a pass band, the interdigital transducer 2-a on the input side is represented by an equivalent circuit shown in FIG. 4(a). Here, a capacitor 9 corresponds to a static capacitance of the interdigital transducer 2-a on the input side. A resistor 10 is a radiation resistance. The electric power dissipated in this resistor 10 corresponds to an electric power transformed to a surface acoustic wave. Since the surface acoustic wave is not excited except in the pass band, the interdigital transducer 2-a is represented outside the pass band by an equivalent circuit comprising only a capacitor 9 as shown in FIG. 4(b). Accordingly, the T-type circuit shows a low pass type characteristic at the frequencies other than the above-mentioned frequency. FIG. 3 shows a low pass type characteristic, that is, a power P.sub.E fed to a terminal resistor 8. A cut-off frequency fc, that is, the frequency at which a power P.sub.E drops to the level of 3dB is decided by inductance of the inductor 3 and capacitance of the interdigital transducer 2-a on the input side.
Since, at a frequency lower than the cut-off frequency fc, the T-type circuit is represented by an equivalent circuit shown in FIG. 4(b) outside the pass band of the surface acoustic wave filter 1, an electric power input to the T-type circuit is completely outputted to the terminal resistor 8. In other words, the T-type circuit operates as a simple transmission line. On the other hand, since the T-type circuit is represented by an equivalent circuit FIG. 4(a) at the pass band of the surface acoustic wave filter 1, the level of the electric power P.sub.E decreases slightly. Corresponding to this, FIG. 3 shows electric powers P.sub.1, P.sub.2, and P.sub.3 fed to the load resistor 5. Here, P.sub.1 is the transmitted electric power at the fundamental frequency fo of the surface acoustic wave filter 1 and P.sub.2 is the transmitted electric power due to undesired spurious response caused by bulk waves of the surface acoustic wave filter 1. Power P.sub.2 contains frequency components between fc and a second harmonic 2fo. P.sub.3 is a transmitted electric power due to the third harmonic 3fo.
Next, the case where the above-mentioned T-type circuit of one section is cascade connected in multi-stages to constitute a filter bank as shown will be described. Since each T-type circuit operates as a simple transmission line at frequencies lower than the cut-off frequency fc of the T-type circuit and outside the pass band of the surface acoustic wave filter 1, even if a plurality of T-type circuits composed of surface acoustic wave filters 1 each having a different pass band are cascade connected in multi-stages, the output level at the load resistor 5 at the pass band of each surface acoustic wave filter 1 is nearly equal to that in the case where only one section of the T-type circuit is used.
Now, the filter bank need output only an electric power P.sub.1 at the fundamental frequency of each surface acoustic wave filter 1 and the output levels of other frequency components should be sufficiently low. Accordingly, electric powers P.sub.2 and P.sub.3 are unnecessary spurious components.
Until now, even the filter bank of this kind can only slightly decrease levels of the electric powers P.sub.2 and P.sub.3 using low pass characteristics of the T-type circuit. In other words, by placing the stop band of the T-type circuit on the frequency range of the electric powers P.sub.2 and P.sub.3, the electric powers P.sub.2 and P.sub.3 due to unnecessary spurious responses at the output terminal 4 of the filter bank can be slightly reduced in level compared with the case where a single surface acoustic wave filter not constituting a filter bank is employed.
Since a conventional filter bank is constituted as described above, an amount of attenuation of a transmitted electric power at the stop band per one section of the T-type circuit is reduced. There was, therefore, a problem that it is more difficult for the surface acoustic wave filter 1 having a section closer to the signal source 7 to obtain attenuation effect due to the T-type circuit, and it is nearly impossible to reduce the electric power P.sub.2 and P.sub.3 containing spurious components.